Quality coatings with cold deposition

Above: The Impact cold deposition system (Source: Impact Innovations)

A next-generation cold spraying coating system has been developed with the goal of significantly improving the corrosion resistance of storage and transport containers used in the nuclear industry. These very large and heavy casks are typically stainless steel. Using the cold spray process a nickel-based alloy is sprayed on the inside of the containers to give additional corrosion protection using an automated process.

Developed by Linde working in collaboration with German cold spraying technology firm Impact Innovations, LINSPRAY® Connect complements the Impact EvoCSII cold spray system. The goal of the new system is enhanced reliability and a reduction in the instances of rejects and any rework required.

The quality of the coating technique means that it is possible to use a thinner layer yet still retain very long-lasting corrosion resistance. However, for the nuclear transport sector this approach is used to give very good corrosion resistance but also improved wear characteristics by using a relatively thick coating.

The cold spraying technique has the advantage that it combines the thickness of the coating together with the total density and good bonding with the substrate, in this case the steel of the container. These qualities deliver extremely high corrosion resistance that can withstand all corrosion tests for a considerable period, a key factor for nuclear materials transport.

“That's an advantage as we don't really have any thickness limitation on the deposition process itself. The only limitation on the thickness is there are related costs there as obviously the thicker the coating the more expensive it gets,” explains Linde’s Werner Kro¨mmer. Kro¨mmer, Business Development Engineer for the manufacturing industry segment at Linde, adds: “While quite often we have a requirement to apply a very thin coating that is gas tight, but for the nuclear industry it needs a very robust, thicker coating that can withstand everything for a very, very long time.”

Kro¨mmer continues: “For about 10 years the nuclear industry has been looking for a solution which brings both together corrosion resistance, thick coatings and good bonding to the substrate. While there are some other coating methods that have been tried in the past none have succeeded in meeting all those requirements. We are gaining more and more interest for this application given it has specific advantages.”

The cold spray connection

In the cold deposition process a gas, in this case nitrogen, is heated to about 1000 degrees Celcius or even higher. This hot compressed gas exits through a de Laval nozzle at the tip of the spray gun. That accelerates the gas to extremely high speed but also has a divergent section at the end. As the compressed gas starts expanding it cools down. The metal power is injected into this gas stream and this is used to accelerate metal particles of between 5 to 50 microns to supersonic speeds, typically about three times the speed of sound or between 500 and 1200 metres per second. However, the metal itself typically stays at relatively low temperature and only uses kinetic energy for the deposition process. “We get huge acceleration, but when the gas exits the nozzle with the metal powder itself it's typically below 100 degrees Celsius. That means the metal always stays in a solid- state,” says Kro¨mmer.

This is the key distinguishing characteristic compared with thermal deposition techniques. The actual temperature depends on the material, but in any event, it always remains far below the melting point of the metal itself. In the cold deposition process the metals therefore do not experience any phase changes. They retain their physical and chemical properties and the metal particles remain oxidation resistant during deposition. The coating material is exactly the same as the starting material – a major advantage for these kinds of applications. The coating created via this process is also ductile which prevents any risk of embrittlement under stress. “The beauty of this process is that they can spray pretty much pure metals, that means we don't have any oxidation in the process and this way we get a very even dense coating that’s gas tight,” explains Kro¨mmer.

For this process to be consistent the gas supply requires a constant pressure and relatively high volume and it is also important to have constant quality to be able to control all the process parameters for the entire time. For the cold process there are two main input factors that can change – the pressure and the gas temperature. While the temperature can be controlled within the gun the pressure is determined by the gas supply system, which in this case is supplied by Linde.

“Due to the fact that the complete process is accelerating particles with nitrogen and we don't have a flame, a constant flow is extremely important to the deposition of this unique coating in terms of the thickness and coating density. Any fluctuation in the gas stream results in a different coating quality and therefore it's critical to have very good control of these parameters with constant gas pressure and volume and also the purity of the gas. High purity gas gives a very long lifetime of the gun itself, especially for heating elements,” says Kro¨mmer.

Transport containers are large items that can weigh up to 120 tonnes and which require several hours to apply the coating even using the automated processes. Any failure or variation during the deposition process can mean the coating process is not to specification. “It needs to be stable for the whole production,” says Kro¨mmer, adding: “If you have just the little defect in the coating, then you have to redo the entire job”.

The cold deposition system was developed in partnership with Impact Innovations but Linde supplies the whole gas supply system from the pumps, tank, piping, and through to the specialist regulators which are able to handle large volumes of nitrogen at extremely high pressures of between about 70 and up to 100 bars. The gas supply system is connected to the control unit of the cold spray from Impact and communicates automatically and in real time. In the event of an error message an emergency redundant gas supply can be triggered and that can ensure a continuous gas flow with no pressure fluctuations so that a transport cask can be completed.

“Cold spraying is perhaps the newest coating technology as it's just 25 years old compared to others and it's absolutely new to the nuclear business. With the equipment which is now available we can reach coating performance that would not have been possible 15 years ago. We really have opportunities which we didn't have before.”

Leonhard Holzgassner, Technical Director & Founder, Impact Innovations, noted: “Improving productivity, reducing costs, maintenance and energy usage are never-ending goals for manufacturing innovators. While that is true, perhaps Kro¨mmer is closer to the mark. He concludes: “Cost is not a critical issue in this application but it is quality driven. The nuclear business needs a container that simply lasts for many hundreds or thousand years and this is one of the few ways of doing it.”